1. Field of the Invention
The present invention relates to soft ferromagnetic films and to fabrication processes for manufacturing such films.
2. Description of the Prior Art
By way of background, soft ferromagnetic films are used for a variety of applications where film magnetization switching speed determines device performance. Examples include thin film inductors for RF and microwave circuits, magnetic random access memory arrays (MRAM), and magnetic recording. With respect to the latter category, the issue of fast magnetic switching speed becomes critical in designing write heads capable of operating at high data rates. In particular, the data writing process is enabled by guiding magnetic flux in the yoke portion of the write head to the pole tips of the head. The speed at which the magnetization direction of the pole tips can be reversed sets the limit for the speed of magnetic recording.
FIGS. 1 and 2 illustrate the geometry of a conventional integrated read/write head. The head includes a pair of soft ferromagnetic film layers P1 and P2 that extend from a back gap area BG to an ABS (Air Bearing Surface). There, the P1 and P2 layers respectively form pole tips PT1 and PT2. The pole tips are separated by an insulative gap layer G3 that defines the head""s write gap. An electromagnetic coil structure C is sandwiched between the P1 and P2 layers to define the yoke portion of the head. The yoke extends from the back gap BG to the pole tips PT1 and PT2. Insulative layers I1, I2 and I3 electrically insulate the coil structure C from the P1 and P2 layers. The read portion of the read/write head of FIGS. 1 and 2 lies between a pair of shield layers S1 and S2. Note that the S2 layer is the same layer that forms the P1 layer. This is known as a xe2x80x9cmergedxe2x80x9d design. The S2 and P1 layers can also be formed separately in what is known as a xe2x80x9cpiggybackxe2x80x9d configuration. Located between the S1 and S2 layers is a pair of insulative G1 and G2 gap layers. A read sensor S is located between the G1 and G2 layers at the ABS.
The coil C is electrically driven by a pair of electrical leads E1 and E2. During write operations, electrical current passing through the coil generates a magnetic field that induces a magnetic flux in the P1 and P2 layers. As shown in FIG. 3, this magnetic flux propagates from the yoke to the pole tips PT1 and PT2, where it fringes across the G3 gap layer. This will cause a magnetic domain to be formed on an underlying magnetic recording medium. The orientation of the recorded magnetic domain is dependent on the magnetization direction of the pole tips PT1 and PT2, which in turn is determined by the direction of the electrical current passing through the coil C. Reversing the coil""s electrical current reverses the magnetization direction of the pole tips PT1 and PT2, and consequently reverses the orientation of the next recorded magnetic domain. This magnetization reversal process is used to encode binary data on the recording medium.
Extensive studies have shown that flux propagation and magnetization reversal in a soft ferromagnetic film are greatly influenced by the intrinsic magnetic properties of the film. More specifically, it is known that to achieve fast magnetic flux propagation and magnetization reversal, the magnetic domains in the film must be aligned such that the domain walls (representing the easy axis of magnetization) are perpendicular to the direction of magnetic flux propagation. In a magnetic write head application, this means that the easy axis must be parallel to the ABS. Head manufacturers strive to create this desirable orientation of magnetic domains by applying a large magnetic field (e.g., about 1500 Oe (Oersteds)) during the formation of the P1 and P2 film layers. The applied field is shown in FIG. 4 by way of reference letter xe2x80x9cH.xe2x80x9d The direction of the H field is parallel to the ABS and away from the reader. In theory, when the P1 and P2 layers are formed, their magnetic domains should be aligned in the direction of the H field, which sets the easy axis of the PT1 and PT2 pole tips.
Even though the desired properties of soft ferromagnetic films are known, achieving them given existing constraints on saturation moment, permeability, ability to mass manufacture, and other factors, is difficult in practice. Applicants have observed that one of the limiting factors on fast magnetic switching speed is mechanical stress in the film in combination with high magnetostriction. Even though the stress distribution in a full film may be isotropic, when the film is patterned during fabrication, the stress distribution in the area near the edges of the patterned structure tends to become anisotropic. If the film has high positive or negative magnetostriction, the anisotropic stress distribution translates to magnetic anisotropy depending on whether the film""s intrinsic stress is tensile or compressive. In particular, Applicant""s have observed that undesirable magnetic anisotropy develops under the following conditions: (1) positive magnetostriction coupled with intrinsic tensile stress, and (2) negative magnetostriction coupled with intrinsic compressive stress. Patterning under these conditions causes the magnetic domains to realign the film""s easy axis in a direction that, for practical cases, tends to be generally parallel to the direction of flux propagation. When a patterned soft ferromagnetic film has high aspect ratio (area to perimeter), as is the case in the pole tips of a write head yoke designed for magnetic recording, this effect becomes dominant.
FIGS. 5(a) and 5(b) illustrate this phenomenon relative to a P1 or P2 yoke/pole tip structure. FIG. 5(a) shows the soft ferromagnetic film prior to patterning. The film""s easy axis is shown by the double-headed arrow. Assume that the arrow is aligned parallel to the direction of the ABS to be subsequently formed. FIG. 5(b) shows the same film after patterning to create the yoke/pole tip structure. The single-headed arrows show the orientation of the magnetic domains, and thus represent the easy axis direction at various locations in the structure. Note that the edge stress anisotropy has changed the easy axis direction parallel to the edges. In the pole tip portion of the structure, the easy axis is predominantly perpendicular to the ABS.
Most soft ferromagnetic films used for magnetic write head yoke structures are electroplated. This process tends to produce tensile stress in the full film. Patterning of such films typically results in the formation of 2-10 um wide regions with high stress anisotropy near the edges. This is due to the fact that an edge cannot react tensile forces perpendicularly thereto, such that there is stress relief in that direction in the immediate vicinity of the edge. On the other hand, tensile forces parallel to the edge can be reacted, and there will be no stress relief in that direction. A positive stress anisotropy condition thus develops.
FIG. 6(a) shows a simulated stress distribution for a soft ferromagnetic film formed using a conventional electroplating process, followed by patterning. The x axis shows distance from the center of the film (x=0) measured in microns (xcexcm). The y axis shows stress anisotropy measured in Mega-Pascals (MPa). The stress anisotropy represents the maximum stress differential along two mutually orthogonal directions in the film. A stress anisotropy value of zero signifies that the stress level is the same in all directions. A positive stress anisotropy value means that the stress differential is positive. A negative stress anisotropy value means that the stress differential is negative. FIG. 6(a) shows that there is zero stress anisotropy throughout the central region of the patterned film. The positive stress anisotropy on each side of the graph of FIG. 6(a) represents a positive stress differential at the edges of the patterned film due to the tensile stress relief condition discussed above.
When the stress distribution shown in FIG. 6(a) is combined with positive magnetostriction of the patterned film, the magnetic anisotropy distribution of FIG. 6(b) results. In FIG. 6(b), the x axis shows distance from the center of the film (x=0) measured in microns (xcexcm). The y axis shows the anisotropy field Hk measured in Oersteds. As is well known, the anisotropy field Hk represents the amount of applied magnetic field required to shift the magnetic moment associated with the magnetic domains of a ferromagnetic material 90 degrees from the easy axis orientation. If the magnetic anisotropy value of a film is zero, it has no easy axis. Positive and negative values signify the existence of an easy axis having some given direction.
In FIG. 6(b), sign of the magnetic anisotropy changes when moving to the patterned edge of the film. This indicates that the easy axis has shifted from its initial orientation. In practice, this effect can be observed by examining the domain structures in patterned electroplated films using Kerr domain imaging. Examining the practical geometry of a magnetic write head yoke, one can be convinced that this phenomenon is leading to the magnetic domain structures shown in FIG. 5(b), which results in slow magnetic switching.
An improved soft ferromagnetic film fabrication method is required if improvements in the magnetization switching speed are to be realized for magnetic write head yokes and other devices. What is needed is a new fabrication method wherein edge stress anisotropy and consequent easy axis magnetization misalignments are avoided.
The foregoing problems are solved and an advance in the art is obtained by a novel method for fabricating a soft ferromagnetic film structure with controlled edge stress anisotropy and enhanced magnetization switching speed. According to the method, a soft ferromagnetic film structure is formed over an underlying structure. The soft ferromagnetic film structure has one or more edges exhibiting edge stress anisotropy. A non-ferromagnetic film structure is formed along the one or more edges of the soft ferromagnetic film structure to induce stress contributions therein. This modifies the edge stress anisotropy in the patterned film by either eliminating it or changing its sign to align the magnetic anisotropy in a preferred way by inducing stress contributions higher than those which are present in the patterned film prior to application of the non-ferromagnetic film. The stress contributions can be supplied by the initial stress anisotropy of the non-ferromagnetic film as deposited, and can additionally be supplied by stress changes during processing steps such as higher temperature annealing.
In embodiments of the invention directed to magnetic write heads, the soft ferromagnetic film structure includes a transition metal alloy, such as a material from the group consisting of alloys of nickel-iron (permalloy), nickel-iron-cobalt alloys, Sendust and cobalt-rare earth alloys. The non-ferromagnetic film structure is made from a material that is sufficiently stiff to react the edge stress anisotropy away from the edges of the soft ferromagnetic film structure or even reverse the edge stress anisotropy along such edges. In can be a non-ferromagnetic metal or a non-metallic electrical insulator material.
The soft ferromagnetic film structure and the non-ferromagnetic film structure can be formed using an electroplating process, a vacuum deposition process, or a combination of both. If an electroplating process is used, the non-ferromagnetic film structure will comprise a non-ferromagnetic metal capable of being electroplated, such as a material from the group consisting of palladium, copper and nickel-phosphorus alloy. If the non-ferromagnetic film structure is deposited, it may either be a metal or a non-metal, such as a material from the group consisting of Al2O3 and SiO2.
In preferred applications, the soft ferromagnetic film structure is formed as a magnetic write head yoke/pole tip structure of an integrated read/write transducer. The invention further contemplates a disk drive that contains a magnetic write head component made in accordance with the inventive process. The invention may also be used to produce MRAM devices and thin film inductors for RF and microwave circuits.